Descending pathways in motor control

Amyotrophic lateral sclerosis represents corticomotoneuronal system failure

Several decades have passed since the anterograde corticomotoneuronal hypothesis for amyotrophic lateral sclerosis (ALS) was proposed. The intervening years have witnessed its emergent support based on anatomical, pathological, physiological, neuroimaging, and molecular biological studies. The evolution of an extensive corticomotoneuronal system appears restricted to the human species, with ALS representing a uniquely human disease. While some, very select non-human primates have limited corticomotoneuronal projections, these tend to be absent in all other animals. From a general perspective, the early clinical features of ALS may be considered to reflect failure of the corticomotoneuronal system. The characteristic loss of skilled motor dexterity involving the limbs, and speech impairment through progressive bulbar dysfunction specifically involve those motor units having the strongest corticomotoneuronal projections. A similar explanation likely underlies the unique "split phenotypes" that have now been well characterized in ALS. Large Betz cells and other pyramidal corticomotoneuronal projecting neurons, with their extensive dendritic arborization, are particularly vulnerable to the elements of the ALS exposome such as aging, environmental stress and lifestyle changes. Progressive failure of the proteosome impairs nucleocytoplasmic shuffling and induces toxic but soluble TDP-43 to aggregate in corticomotoneurons. Betz cell failure is further accentuated through dysfunction of its profuse dendritic arborizations. Clarification of system specific genomes and neural networks will likely promote the initiation of precision medicine approaches directed to support the key structure that underlies the neurological manifestations of ALS, the corticomotoneuronal system.

The deltoid muscle and the pattern of paresis in ALS

There is neuroanatomical and clinical evidence that the corticospinal tract governs the patterns of pareses in sporadic ALS. These patterns are mirrored by phylogenetically young monosynaptic corticomotor neuronal connections. It is well known that, clinically, dysfunction of the deltoid muscle contributes considerably to the early disability of the ALS patient. In this study, we prospectively compared the degree of pareses of the deltoid muscle with the triceps and biceps brachii in N = 71 patients (426 muscles). We could show that the extent of involvement of the deltoid muscle early in the disease process resembles that of the biceps rather than the triceps brachii. This pattern is consistent with functional data of the corticospinal monosynaptic connectivity of all three muscles.

The effects of bilateral M1 anodal tDCS on corticomotor excitability and acquisition the of a bimanual videogame skill

Most activities of daily life involve some degree of coordinated, bimanual activity from the upper limbs. However, compared to single-handed movements, bimanual movements are processed, learned, and controlled from both hemispheres of the brain. Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that enhances motor learning by modulating the activity of movement-associated brain regions. While effective in simple, single-handed tasks, tDCS has shown mixed results in complex bimanual tasks. This study investigated the effects of bilateral M1 anodal tDCS (biM1 a-tDCS) on learning and cortical excitability during a customized, bimanual racing videogame task. Thirty-six right-handed adults completed three lab visits (∼48 h apart), practicing the task while receiving either biM1 a-tDCS or SHAM tDCS. Cortical excitability was measured with transcranial magnetic stimulation (TMS) and electromyography (EMG) before and after the first visit. Though all subjects demonstrated improvements over the course of the study, our analyses revealed significantly faster rates of learning on days 1 & 2, but not day 3, of practice in subjects receiving biM1 a-tDCS. Moreover, perhaps due to differences in baseline gaming experience and aptitude, this effect appeared to be stronger in female subjects. Interestingly, no significant differences in corticomotor excitability were observed between conditions. Though biM1 a-tDCS did not appear to impact corticomotor excitability, our results contribute to the growing body of evidence which seems to suggest that multifocal tDCS protocols may be superior to traditional, single-site tDCS for the enhancement of bimanual motor learning.

Soma and neurite density imaging detects brain microstructural impairments in amyotrophic lateral sclerosis

OBJECTIVE

To investigate whole-brain microstructural changes in amyotrophic lateral sclerosis (ALS) using soma and neurite density imaging (SANDI), a novel multicompartment model of diffusion-weighted imaging that estimates apparent soma and neurite density.

METHODS

This study consists of 41 healthy controls and 43 patients with ALS, whose diffusion-weighted data were acquired. The SANDI-derived (including signal fractions of soma (fsoma), neurite (fneurite), and extra-cellular space (fextra)) and diffusion tensor imaging (DTI)-derived metrics were obtained. Voxel-based analyses were performed to evaluate intergroup differences and the correlation of SANDI and DTI metrics with clinical parameters.

RESULTS

In ALS patients, fneurite reduction involved both gray matter (primarily the bilateral precentral gyri, supplementary motor area, medial frontal gyrus, anterior cingulate cortex, inferior frontal gyrus, orbital gyrus, paracentral lobule, postcentral gyrus, middle cingulate cortex, hippocampus and parahippocampal gyrus, and insula, and left anterior parts of the temporal lobe) and white matter (primarily the bilateral corticospinal tract, body of corpus callosum, and brainstem) (P <0.05 after false discovery rate correction). The fextra increment showed a similar spatial distribution in ALS patients. Interestingly, the decreased fsoma in ALS primarily located in gray matter; while, the increased fsoma primarily involved white matter. The spatial distribution of fneurite/fextra/fsoma changes was larger than that detected by conventional DTI metrics, and the fneurite/fextra/fsoma were correlated with disease severity.

CONCLUSIONS

SANDI may serve as a clinically relevant model, superior to conventional DTI, for characterizing microstructural impairments such as neurite degeneration and soma alteration in ALS.

Effector-dependent decline in strength and subcortical motor excitability with aging

A decline in upper limb strength is common with normal aging. However, whether age-related strength decline is paralleled by reduced excitability of descending motor pathways is unclear. The reticulospinal tract is a key subcortical pathway involved in gross motor output and exhibits increased excitability following resistance training. Here, we sought to determine age-related effects on strength and reticulospinal excitability in flexors and extensors of the upper limb in humans. In 15 younger and 14 older adults, we quantified upper limb strength using dynamometry, and reticulospinal excitability by using transcranial magnetic stimulation to elicit ipsilateral motor evoked potentials. We observed a decline in flexion, but not extension strength, in older compared with younger adults. This behavioral pattern was paralleled by an age-related reduction in ipsilateral motor evoked potential presence specific to flexor muscles. Our findings indicate that reduced excitability of the reticulospinal tract, which exhibits strong innervation of flexor muscles, may be a key contributor to upper limb strength decline commonly observed in older adults.

Single-Nuclei Sequencing Reveals a Robust Corticospinal Response to Nearby Axotomy But Overall Insensitivity to Spinal Injury

The ability of neurons to sense and respond to damage is crucial for maintaining homeostasis and facilitating nervous system repair. For some cell types, notably dorsal root ganglia and retinal ganglion cells, extensive profiling has uncovered a significant transcriptional response to axon injury, which influences survival and regenerative outcomes. In contrast, the injury responses of most supraspinal cell types, which display limited regeneration after spinal damage, remain mostly unknown. In this study, we used single-nuclei sequencing in adult male and female mice to profile the transcriptional responses of diverse supraspinal cell types to spinal injury. Surprisingly, thoracic spinal injury induced only modest changes in gene expression across all populations, including corticospinal tract (CST) neurons. Additionally, CST neurons exhibited minimal response to cervical injury but showed a much stronger reaction to intracortical axotomy, with upregulation of numerous regeneration and apoptosis-related transcripts shared with injured DRG and RGC neurons. Thus, the muted response of CST neurons to spinal injury is linked to the injury's distal location, rather than intrinsic cellular characteristics. More broadly, these findings indicate that a central challenge for enhancing regeneration after a spinal injury is the limited detection of distant injuries and the subsequent modest baseline neuronal response.

A brain-wide map of descending inputs onto spinal V1 interneurons

Motor output results from the coordinated activity of neural circuits distributed across multiple brain regions that convey information to the spinal cord via descending motor pathways. Yet the organizational logic through which supraspinal systems target discrete components of spinal motor circuits remains unclear. Here, using viral transsynaptic tracing along with serial two-photon tomography, we have generated a whole-brain map of monosynaptic inputs to spinal V1 interneurons, a major inhibitory population involved in motor control. We identified 26 distinct brain structures that directly innervate V1 interneurons, spanning medullary and pontine regions in the hindbrain as well as cortical, midbrain, cerebellar, and neuromodulatory systems. Moreover, we identified broad but biased input from supraspinal systems onto V1Foxp2 and V1Pou6f2 neuronal subsets. Collectively, these studies reveal elements of biased connectivity and convergence in descending inputs to molecularly distinct interneuron subsets and provide an anatomical foundation for understanding how supraspinal systems influence spinal motor circuits.

Conjoint specification of action by neocortex and striatum

The interplay between two major forebrain structures-cortex and subcortical striatum-is critical for flexible, goal-directed action. Traditionally, it has been proposed that striatum is critical for selecting what type of action is initiated, while the primary motor cortex is involved in specifying the continuous parameters of an upcoming/ongoing movement. Recent data indicate that striatum may also be involved in specification. These alternatives have been difficult to reconcile because comparing very distinct actions, as is often done, makes essentially indistinguishable predictions. Here, we develop quantitative models to reveal a somewhat paradoxical insight: only comparing neural activity across similar actions makes strongly distinguishing predictions. We thus developed a novel reach-to-pull task in which mice reliably selected between two similar but distinct reach targets and pull forces. Simultaneous cortical and subcortical recordings were uniquely consistent with a model in which cortex and striatum jointly specify continuous parameters governing movement execution.

Effects of lumbar joint mobilization on trunk control, balance, and gait in patients with stroke: study protocol for a randomized controlled trial

BACKGROUND

After stroke, most patients have impairments in trunk control, balance, and gait. These dysfunctions are closely related to the ability to perform activities of daily living. Therefore, restoring these functions has become the primary rehabilitation goal for stroke patients. Lumbar joint mobilization can activate the spine and surrounding muscles, increase the sensory perception of lumbar joints, restore the stability and symmetry of the trunk. Few studies have focused on the use of lumbar joint mobilization in stroke patients. The purpose of this study was to explore the effects of lumbar joint mobilization on trunk control, balance, and gait in stroke patients.

METHODS

Sixty stroke patients will be recruited from the Department of Rehabilitation Medicine, Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology. The participants will be randomly divided into a control group (n = 30) and an intervention group (n = 30). Both groups will receive conventional physical therapy once a day for 4 weeks, 5 days a week. The intervention group will receive an additional 10 min of lumbar joint mobilization at the end of each conventional physical therapy session. The primary outcome measure is the Trunk Impairment Scale (TIS), the secondary outcome measures are Berg Balance Scale (BBS), Time Up & Go test (TUG) and Functional Gait Assessment (FGA). Patients will be assessed for the outcome measures at baseline and at the 4th week at the end of treatment.

DISCUSSION

The results of this study will provide preliminary data on the efficacy of lumbar joint mobilization combined with conventional physical therapy on trunk control, balance, and gait in stroke patients.

TRIAL REGISTRATION

The trial was registered in the Chinese Clinical Trial Registry on September 3, 2023, with the registration number ChiCTR2300075377. The URL of trial registry record: https://www.chictr.org.cn/showproj.html?proj=206313 .

Cervical spinal cord stimulation for treatment of upper limb paralysis: a narrative review

Recent advances in cervical spinal cord stimulation (SCS) have demonstrated improved efficacy as a therapeutic intervention for restoring hand functions in individuals with spinal cord injuries or stroke. Accumulating evidence consistently shows that cervical SCS yields significant improvements in grip force, proximal arm strength and muscle activation, with both immediate and sustained effects. This review synthesizes the evidence that electrical stimulations modulate the spinal and supraspinal organization of uninjured descending motor tracts, primarily the residual corticospinal tract, reticulospinal tract and propriospinal network of neurons, as well as increasing the sensitivity of spinal interneurons at the stimulated segments to these inputs. Additionally, we examine contemporary strategies aimed at achieving more precise patterned stimulations, including intraspinal microstimulation, ventral cord stimulation and closed-loop neuromodulation, and discuss the potential benefits of incorporating cervical SCS into a multimodal treatment paradigm.Level of evidence: V.

From non-human to human primates: a translational approach to enhancing resection, safety, and indications in glioma surgery while preserving sensorimotor abilities

Since the pivotal studies of neurophysiologists in the early 20th century, research on brain functions in non-human primates has provided valuable insights into the neural mechanisms subserving neurological function. By using data acquired on non-human primates as a reference, important progress in knowledge of the human brain and its functions has been achieved. The translational impact allowed by this scientific effort must be recognized in the implementation of the current surgical techniques particularly in support of the neurosurgical approach to brain tumors. In the surgical treatment of brain tumors, the ability to maximally extend the resection allows an improvement in overall survival, progression-free survival, and quality of life of patients. The main goal, and, at the same time, the main challenge, of oncological neurological surgery is to avoid permanent neurological deficit while reaching maximal resection, particularly when the tumor infiltrates the neural network subserving motor functions. Brain mapping techniques were developed using neurophysiological probes to identify the areas and tracts subserving sensorimotor function, ensuring their preservation during the resection. During the last 20 years, starting from the classical "Penfield" technique, brain mapping has been progressively implemented. Among the major advancements was the introduction of high-frequency direct electrical stimulation. Its refinement, along with the complementary use of low-frequency stimulation, allowed a further refinement of stimulation protocols. In this narrative review, we propose an analysis of the process through which the knowledge acquired through experiments on non-human primates influenced and changed the current approach to neurosurgical procedures. We then describe the main brain mapping techniques used in the resection of tumors located within sensorimotor circuits. We also detail how these techniques allowed the acquisition of new data on the properties of areas and tracts underlying sensorimotor control, in turn fostering the design of new tools to navigate within cortical and subcortical areas, that were before deemed to be "sacred and untouchable."

Brainwave Activity Localization, Mood Symptoms, and Balance Impairment in a Male South African Rugby Player With Persisting Symptoms After Concussion: A Case Report

The case sets the foundation for clinical protocols to incorporate mobile EEG and qEEG techniques, instrumental balance testing, and mood symptom screening in athletes who have suffered a sports-related concussion. The protocol provides a framework for clinicians to monitor a patient's recovery progress in terms of brainwave activity, general cognition, moods, and motor control. Objective data obtained through the protocol may assist in developing personalized treatment plans, improving follow-up care, and identifying residual brain function deficits that may be missed in standardized clinical exams. Finally, this case highlights a need for more thorough communication and testing procedures that screen for mood symptoms and provide an opportunity for athletes to discuss their mental health after suffering from an SRC.

Age-related differences in agility are related to both muscle strength and corticospinal tract function

Agility is essential for "healthy" aging, but neuromuscular contributions to age-related differences in agility are not entirely understood. We recruited healthy (n = 32) non-athletes (30-84 years) to determine: (1) if aging is associated with agility and (2) whether muscle strength or corticospinal tract function predicts agility. We assessed muscle strength via a validated knee extension test, corticospinal tract function via transcranial magnetic stimulation, and agility via spatiotemporal values (i.e., leg length-adjusted hop length and hop length variability) collected during a novel propulsive bipedal hopping (agility) task on an electronic walkway. Pearson correlation revealed aging is associated with leg length-adjusted hop length (r = -0.671, p < 0.001) and hop length variability (r = 0.423, p = 0.016). Further, leg length-adjusted hop length and hop length variability correlated with quadriceps strength (r = 0.581, p < 0.001; r = -0.364, p = 0.048) and corticospinal tract function (r = -0.384, p = 0.039; r = 0.478, p = 0.007). However, hierarchical regressions indicated that, when controlling for sex, muscle strength only predicts leg length-adjusted hop length (R2 = 0.345, p = 0.002), whereas corticospinal tract function only predicts hop length variability (R2 = 0.239, p = 0.014). Therefore, weaker quadriceps decrease the distance hopped, and deteriorating corticospinal tract function increases variability in hop length.

Structure-function coupling alterations in cognitively normal individuals with white matter hyperintensities

BACKGROUND

White matter hyperintensities (WMH) are prominent neuroimaging markers of cerebral small vessel disease (CSVD) linked to cognitive decline. Nevertheless, the pathophysiological mechanisms underlying WMH remain unclear.

OBJECTIVE

This study aimed to assess the structural decoupling index (SDI) as a novel metric for quantifying the brain's hierarchical organization associated with WMH in cognitively normal older adults.

METHODS

We analyzed data from 112 cognitively normal individuals with varying WMH burdens (43 high WMH burden and 69 low WMH burden). Neuroimaging data were used to calculate SDI, and gene enrichment analysis was conducted to explore related molecular pathways.

RESULTS

An increased spatial gradient of SDI from the sensory-motor cortex to the associative cortex was observed. Compared to the low WMH burden group, the high WMH group exhibited elevated SDI in the right superior frontal gyrus, bilateral orbital gyrus, bilateral precentral gyrus, bilateral cingulate gyrus, bilateral thalamus, and bilateral striatum. In the high WMH burden group, SDI in the left thalamus and right cingulate gyrus negatively correlated with memory, while SDI in the right orbital gyrus and left precentral gyrus positively correlated with processing speed. Gene enrichment analysis highlighted associations with pathways involved in neural system function, potassium ion transmembrane transport, synaptic signaling, neuron projection development, and cell secretion regulation.

CONCLUSIONS

The findings suggest SDI alterations as a potential mechanistic pathway in WMH, which is associated with significant molecular pathways and cognitive impairments. This study provides a theoretical framework for understanding the pathophysiology of WMH progression and subsequent cognitive deficits.

In Vivo Assessment of Cortical Astrocyte Network Dysfunction During Autoimmune Demyelination: Correlation With Disease Severity

Cortical damage and dysfunction is a pathological hallmark of multiple sclerosis (MS) that correlates with the severity of physical and cognitive disability. Astrocytes participate in MS pathobiology through a variety of mechanisms, and abnormal astrocytic calcium signaling has been pointed as a pathogenic mechanism of cortical dysfunction in MS. However, in vivo evidence supporting deregulation of astrocyte calcium-dependent mechanisms in cortical MS is still limited. Here, we applied fiber photometry to the longitudinal analysis of spontaneous and sensory-evoked astrocyte network activity in the somatosensory cortex of mice in an experimental autoimmune encephalomyelitis (EAE). We found that freely moving EAE mice exhibit spontaneously occurring astrocyte calcium signals of increased duration and reduced amplitude. Concomitantly, cortical astrocytes in EAE mice responded to sensory stimulation with calcium events of decreased amplitude. The emergence of aberrant astrocyte calcium signals in the somatosensory cortex paralleled the onset of neurological symptomatology, and changes in the amplitude of both spontaneous and evoked responses were selectively correlated to the severity of neurological deficits. These results highlight the imbalance of astrocyte network activity in the brain cortex during autoimmune inflammation and further support the relevance of astrocyte-based pathobiology as an underlying mechanism of cortical dysfunction in MS.

Vagus nerve stimulation for stroke rehabilitation: Neural substrates, neuromodulatory effects and therapeutic implications

Paired vagus nerve stimulation (VNS) has emerged as a promising strategy to potentiate recovery after neurological injury. This approach, which combines short bursts of electrical stimulation of the vagus nerve with rehabilitation exercises, received approval from the US Food and Drug Aministration in 2021 as the first neuromodulation-based therapy for chronic stroke. Because this treatment is increasingly implemented in clinical practice, there is a need to take stock of what we know about this approach and what we have yet to learn. Here, we provide a survey on the foundational basis of VNS therapy for stroke and offer insight into the mechanisms that underlie potentiated recovery, focusing on the principles of neuromodulatory reinforcement. We discuss the current state of observations regarding synaptic reorganization in motor networks that are enhanced by VNS, and we propose other prospective loci of neuromodulation that should be evaluated in the future. Finally, we highlight the future opportunities and challenges to be faced as this approach is increasingly translated to clinical use. Collectively, a clearer understanding of the mechanistic basis of VNS therapy may reveal ways to maximize its benefits.

Pharmacological blocking of spinal GABAA receptors in monkeys reduces sensory transmission to the spinal cord, thalamus, and cortex

A century of research established that GABA inhibits proprioceptive inputs presynaptically to sculpt spinal neural inputs into skilled motor output. Recent results in mice challenged this theory by showing that GABA can also facilitate action potential conduction in proprioceptive afferents. Here, we tackle this controversy in monkeys, the most human-relevant animal model, and show that GABAA receptors (GABAARs) indeed facilitate sensory inputs to spinal motoneurons and interneurons and that this mechanism also influences sensory transmission to supraspinal centers. We performed causal manipulations of GABAARs with intrathecal pharmacology in anesthetized monkeys while recording electrical signals in the muscles, spinal cord, thalamus, and cortex. We show that blocking GABAARs suppresses spinal reflexes to hand muscles, sensory-evoked single-unit firing in the spinal cord, and sensory-evoked potentials in the thalamus and somatosensory cortex. Our results portray a sophisticated picture of presynaptic modulation of sensory inputs by GABA in the spinal cord.

Central projections of nociceptive input originating from the low back and limb muscle in rats

Since clinical features of chronic muscle pain originating from the low back and limbs are different (higher prevalence and broader/duller sensation of low back muscle pain than limb muscle pain), spinal and/or supraspinal projection of nociceptive information could differ between the two muscles. We tested this hypothesis using c-Fos immunohistochemistry combined with retrograde-labeling of dorsal horn (DH) neurons projecting to ventrolateral periaqueductal grey (vlPAG) or ventral posterolateral nucleus of the thalamus (VPL) by fluorogold (FG) injections into the vlPAG or VPL. C-Fos expression in the DH was induced by injecting 5% formalin into the multifidus (MF, low back) or gastrocnemius-soleus (GS, limb) muscle. A double-labeled DH neuron showing both c-Fos-immunoreactive nucleus and retrogradely transported FG in the cytoplasm was considered as a nociceptive projection neuron. Consistent with DH somatotopy for proximal vs. distal cutaneous inputs, DH neurons with MF input were located in the most lateral area of laminae I - II (segments Th12 - L5), while those with GS input were located in the middle area of laminae I - II (L3 - L5). DH neurons projecting to the vlPAG were located in superficial DH, while those projecting to VPL were located in deep DH. Supraspinal projection derived from more spinal segments for MF input than for GS input. These data suggest that nociceptive input from low back muscles is integrated more in craniocaudal direction than for limb muscles, and that these signals are then forwarded to both PAG and thalamus and contribute to the different nature of muscle pain arising from the low back and limbs.

Engaging dystonia networks with subthalamic stimulation

Deep brain stimulation is an efficacious treatment for dystonia. While the internal pallidum serves as the primary target, recently, stimulation of the subthalamic nucleus (STN) has been investigated. However, optimal targeting within this structure and its surroundings have not been studied in depth. Indeed, historical targets that have been used for surgical treatment of dystonia are directly adjacent to the STN. Further, multiple types of dystonia exist, and outcomes are variable, suggesting that not all types would profit maximally from the same target. Therefore, a thorough investigation of neural substrates underlying stimulation effects on dystonia signs and symptoms is warranted. Here, we analyze a multicenter cohort of isolated dystonia patients with subthalamic implantations (N = 58) and relate their stimulation sites to improvements of appendicular and cervical symptoms as well as blepharospasm. Stimulation of the ventral oral posterior nucleus of thalamus and surrounding regions were associated with improvements in cervical dystonia, while stimulation of the dorsolateral STN was associated with improvements in limb dystonia and blepharospasm. This dissociation was matched by structural connectivity analysis, where the cerebellothalamic, corticospinal, and pallidosubthalamic tracts were associated with improvements of cervical dystonia, while hyperdirect and subthalamopallidal pathways with alleviation of limb dystonia and blepharospasm. On the level of functional networks, improvements of limb dystonia were associated with connectivity to the corresponding somatotopic regions in the primary motor cortex, while alleviation of cervical dystonia to the cingulo-opercular network. These findings shed light on the pathophysiology of dystonia and may guide DBS targeting and programming in the future.

Selective direct motor cortical influence during naturalistic climbing in mice

It remains poorly resolved when and how motor cortical output directly influences limb muscle activity through descending projections, which impedes mechanistic understanding of cortical movement control. Here we addressed this in mice performing an ethologically inspired all-limb climbing behavior. We quantified the direct influence of forelimb primary motor cortex (caudal forelimb area, CFA) on muscle activity across the muscle activity states that occur during climbing. We found that CFA instructs muscle activity pattern, mainly by selectively activating certain muscles while exerting much smaller, bidirectional effects on their antagonists. From Neuropixel recordings, we identified linear combinations (components) of motor cortical activity that covary with these effects, finding that these components differ partially from those that covary with muscle activity and differ almost completely from those that covary with kinematics. Collectively, our results reveal an instructive direct motor cortical influence on limb muscles that is selective within a motor behavior and reliant on a distinct neural activity subspace.

High-density multielectrode arrays bring cellular resolution to neuronal activity and network analyses of corticospinal motor neurons

Corticospinal motor neurons (CSMN), located in the motor cortex of the brain, are one of the key components of the motor neuron circuitry. They are in part responsible for the initiation and modulation of voluntary movement, and their degeneration is the hallmark for numerous diseases, such as amyotrophic lateral sclerosis (ALS), hereditary spastic paraplegia, and primary lateral sclerosis. Cortical hyperexcitation followed by in-excitability suggests the early involvement of cortical dysfunction in ALS pathology. However, a high-spatiotemporal resolution on our understanding of their functional health and connectivity is lacking. Here, we combine optical imaging with high-density microelectrode array (HD-MEA) system enabling single cell resolution and utilize UCHL1-eGFP mice to bring cell-type specificity to our understanding of the electrophysiological features of healthy CSMN, as they mature and form network connections with other cortical neurons, in vitro. This novel approach lays the foundation for future cell-type specific analyses of CSMN that are diseased due to different underlying causes with cellular precision, and it will allow the assessment of their functional response to compound treatment, especially for drug discovery efforts in upper motor neuron diseases.

VTA projections to M1 are essential for reorganization of layer 2-3 network dynamics underlying motor learning

The primary motor cortex (M1) is crucial for motor skill learning. Previous studies demonstrated that skill acquisition requires dopaminergic VTA (ventral-tegmental area) signaling in M1, however little is known regarding the effect of these inputs at the neuronal and network levels. Using dexterity task, calcium imaging, chemogenetic inhibiting, and geometric data analysis, we demonstrate VTA-dependent reorganization of M1 layer 2-3 during motor learning. While average activity and average functional connectivity of layer 2-3 network remain stable during learning, activity kinetics, correlational configuration of functional connectivity, and average connectivity strength of layer 2-3 neurons gradually transform towards an expert configuration. Additionally, sensory tone representation gradually shifts to success-failure outcome signaling. Inhibiting VTA dopaminergic inputs to M1 during learning, prevents all these changes. Our findings demonstrate dopaminergic VTA-dependent formation of outcome signaling and new connectivity configuration of the layer 2-3 network, supporting reorganization of the M1 network for storing new motor skills.

Claire M, Whitehead SS, Kanakabandi K, Sheng ZM, Xiao Y, Kash JC, Taubenberger JK, Martens C, Cohen JI. Spatiotemporal profile of an optimal host response to virus infection in the primate central nervous system

Viral infections of the central nervous system (CNS) are a major cause of morbidity largely due to lack of prevention and inadequate treatments. While mortality from viral CNS infections is significant, nearly two thirds of the patients survive. Thus, it is important to understand how the human CNS can successfully control virus infection and recover. Since it is not possible to study the human CNS throughout the course of viral infection at the cellular level, here we analyzed a non-lethal viral infection in the CNS of nonhuman primates (NHPs). We inoculated NHPs intracerebrally with a high dose of La Crosse virus (LACV), a bunyavirus that can infect neurons and cause encephalitis primarily in children, but with a very low (≤ 1%) mortality rate. To profile the CNS response to LACV infection, we used an integrative approach that was based on comprehensive analyses of (i) spatiotemporal dynamics of virus replication, (ii) identification of types of infected neurons, (iii) spatiotemporal transcriptomics, and (iv) morphological and functional changes in CNS intrinsic and extrinsic cells. We identified the location, timing, and functional repertoire of optimal transcriptional and translational regulation of the primate CNS in response to virus infection of neurons. These CNS responses involved a well-coordinated spatiotemporal interplay between astrocytes, lymphocytes, microglia, and CNS-border macrophages. Our findings suggest a multifaceted program governing an optimal CNS response to virus infection with specific events coordinated in space and time. This allowed the CNS to successfully control the infection by rapidly clearing the virus from infected neurons, mitigate damage to neurophysiology, activate and terminate immune responses in a timely manner, resolve inflammation, restore homeostasis, and initiate tissue repair. An increased understanding of these processes may provide new therapeutic opportunities to improve outcomes of viral CNS diseases in humans.

Efficacy of Transcranial Magnetic Stimulation in Post-Stroke Motor Recovery: Impact of Impairment Severity

Stroke is a leading cause of impairment, with 70% of survivors experiencing upper limb motor deficits. While transcranial magnetic stimulation (TMS) is widely used in rehabilitation, the impact of impairment severity on treatment outcomes remains unclear. This study evaluated TMS effectiveness in post-stroke motor impairment and explored its neural mechanisms. Fifty-five stroke patients were divided into TMS (n =27) and control (n =28) groups. The TMS group received two weeks of intermittent theta-burst stimulation (iTBS), while controls received sham stimulation. Patients were stratified into mild/moderate (Fugl-Meyer Assessment [FMA] ) and severe (FMA <30) impairment subgroups. Motor function and electroencephalography (EEG) metrics were assessed before and after treatment. Overall FMA improvement showed no difference between groups, but the TMS-mild/moderate impairment group demonstrated significantly greater improvement compared to others. This group exhibited higher global and local alpha band power and global alpha efficiency. FMA improvement positively correlated with local alpha power changes. TMS of ipsilesional M1 improves motor function in mild/moderate impairments but shows limited efficacy in severe cases. EEG suggests TMS promotes recovery by modulating alpha activity and enhancing network efficiency. These findings support stratified treatment approaches and highlight the need for alternative interventions in severe impairment.

Corticomuscular coherence existed at the single motor unit level

The monosynaptic cortico-motoneuronal connections suggest the possibility of individual motor units (MUs) receiving independent commands from motor cortex. However, previous studies that used corticomuscular coherence (CMC) between electroencephalogram (EEG) signals and electromyogram (EMG) signals have not directly explored the corticospinal functionality at the single motoneuron level. The objective of this study is to find out whether synchronous activities exist between the motor cortex and individual MUs. Corticomuscular coherence was calculated between the EEG signals and the MU firing event trains which were extracted using the EMG decomposition technique. The results showed that some but not all MUs indeed had significant coherent activities with the contralateral motor cortex, which we named the cortico-motoneuronal coherence (CMnC). In contrast to the CMC only occurring in β and γ bands, CMnC occurred across the four common EEG frequency bands (θ, α, β and γ). Further, we identified individual MUs that showed significant interactions with the motor cortex. These coherent MUs (CohMU) could still be found even when the EMG signals were not coupled with the cortical activities. Compared with conventional CMC, our preliminary results indicated that the CMnC could potentially help to investigate the complex coupling between cortical and muscular activities due to its ability to separate different correlated components. This study proves that corticomuscular coherence exists at a single MU level, which provides a new perspective for the research on corticomuscular coupling. Further study on the CMnC could help deepen our understanding of the neural control of movement.

Shapes of direct cortical responses vs. short-range axono-cortical evoked potentials: The effects of direct electrical stimulation applied to the human brain

OBJECTIVE

Direct cortical responses (DCR) and axono-cortical evoked potentials (ACEP) are generated by electrically stimulating the cortex either directly or indirectly through white matter pathways, potentially leading to different electrogenic processes. For ACEP, the slow conduction velocity of axons (median ≈ 4 m.s-1) is anticipated to induce a delay. For DCR, direct electrical stimulation (DES) of the cortex is expected to elicit additional cortical activity involving smaller and slower non-myelinated axons. We tried to validate these hypotheses.

METHODS

DES was administered either directly on the cortex or to white matter fascicles within the resection cavity, while recording DCR or ACEP at the cortical level in nine patients.

RESULTS

Short but significant delays (≈ 2 ms) were measurable for ACEP immediately following the initial component (≈ 7 ms). Subsequent activities (≈ 40 ms) exhibited notable differences between DCR and ACEP, suggesting the presence of additional cortical activities for DCR.

CONCLUSION

Distinctions between ACEPs and DCRs can be made based on a delay at the onset of early components and the dissimilarity in the shape of the later components (>40 ms after the DES artifact).

SIGNIFICANCE

The comparison of different types of evoked potentials allows to better understand the effects of DES.

Personalized whole-brain activity patterns predict human corticospinal tract activation in real-time

BACKGROUND

Transcranial magnetic stimulation (TMS) interventions could feasibly treat stroke-related motor impairments, but their effects are highly variable. Brain state-dependent TMS approaches are a promising solution to this problem, but inter-individual variation in lesion location and oscillatory dynamics can make translating them to the poststroke brain challenging. Personalized brain state-dependent approaches specifically designed to address these challenges are needed.

METHODS

As a first step towards this goal, we tested a novel machine learning-based EEG-TMS system that identifies personalized brain activity patterns reflecting strong and weak corticospinal tract (CST) activation (strong and weak CST states) in healthy adults in real-time. Participants completed a single-session study that included the acquisition of a TMS-EEG-EMG training dataset, personalized classifier training, and real-time EEG-informed single-pulse TMS during classifier-predicted personalized CST states.

RESULTS

MEP amplitudes elicited in real-time during classifier-predicted personalized strong CST states were significantly larger than those elicited during corresponding weak and random CST states. MEP amplitudes elicited in real-time during classifier-predicted personalized strong CST states were also significantly less variable than those elicited during corresponding weak CST states. Personalized CST states lasted for ∼1-2 s at a time and ∼1 s elapsed between consecutive similar states. Individual participants exhibited unique differences in spectro-spatial EEG patterns between classifier-predicted personalized strong and weak CST states.

CONCLUSION

Our results show for the first time that personalized whole-brain EEG activity patterns predict CST activation in real-time in healthy humans. These findings represent a pivotal step towards using personalized brain state-dependent TMS interventions to promote poststroke CST function.

Neural structural alterations correlates of quadriceps muscle strength deficits in patients after anterior cruciate ligament reconstruction

Background

Persistent maladaptive changes of corticospinal tract (CST) and quadriceps strength deficits exist in patients with anterior cruciate ligament reconstruction (ACLR). This study aimed to investigate the relationships between the structural alterations of CST and quadriceps muscle strength deficits in patients with ACLR.

Methods

Twenty-nine participants who had undergone unilateral ACLR (29 males; age = 32.61 ± 6.72 years) were enrolled in a cross-sectional investigation. We chose CST as a region of interest and performed diffusion tensor imaging (DTI) that measured the microstructure of white matter tracts. Maximal voluntary isometric quadriceps muscle strength was assessed using a hand-held dynamometer. Simple and partial correlation analyses were performed between the DTI outcomes and quadriceps muscle strength deficits in patients with ACLR before and after controlling for age, sex, BMI, Tegner activity score, and graft type. Sub-group analyses were also performed to investigate the relationships between the DTI outcomes of CST structure and quadriceps muscle strength deficits according to the graft type before and after controlling for age, sex, BMI, and Tegner activity score.

Results

Lower limb symmetry index (LSI) of quadriceps muscle strength was associated with a higher ratio of radial diffusivity (RD, r = -0.379, p = 0.042) in corticospinal tracts of the injured hemisphere to those of the non-injured hemisphere in ACLR patients after controlling for age, BMI, Tegner activity score and graft type. In subgroup analyses of ACLR patients with hamstring autografts, we found that higher injured quadriceps muscle strength was associated with higher axial diffusivity (AD, r = 0.616, p = 0.033) of CST structure and lower LSI of quadriceps muscle strength was associated with higher ratio of mean diffusivity (MD, r = -0.682, p = 0.014) and RD (r = -0.759, p = 0.004) in corticospinal tracts of the injured hemisphere to those of the non-injured hemisphere in ACLR patients after controlling for age, BMI, Tegner activity score.

Conclusion

Decreased integrity (higher ratio of RD) of CST microstructure in ACLR patients was significantly associated with lower quadriceps limb symmetry index, which hinted that quadriceps muscle strength deficits of injured side may be a demyelinating process of CST microstructure in ACLR.

Long-term forced-use therapy after sensorimotor cortex lesions restores contralesional hand function and promotes its preference in Macaca mulatta

Injury to one cerebral hemisphere can result in paresis of the contralesional hand and subsequent preference of the ipsilesional hand in daily activities. However, forced use therapy in humans can improve function of the contralesional paretic hand and increase its use in daily activities, although the ipsilesional hand may remain preferred for fine motor activities. Studies in monkeys have shown that minimal forced use of the contralesional hand, which was the preferred hand prior to brain injury, can produce remarkable recovery of function. Here we tested the hypothesis that long-term forced use of the contralesional hand during the post-lesion period can return it to preferred status. Four rhesus monkeys received tests of hand preference prior to surgical lesions of primary motor cortex, lateral premotor cortex and anterior parietal cortex (F2P2 lesion) contralateral to the preferred hand. Beginning two weeks after the lesion, forced use therapy involving contralateral hand reaches to acquire food targets occurred 3X weekly with at least 300 reaches/session until 24 weeks post-lesion. Despite initial paresis of the contralesional hand, its manipulation skill returned to near pre-lesion levels or higher and all four monkeys returned to a contralesional hand preference late in the post-lesion period. Favorable reorganization of spared cortical and subcortical neural networks may promote recovery of hand function and preference. These results have relevance for the use of extensive forced-use therapy in humans who experience unilateral periRolandic injury to potentially support better recovery of contralesional hand function.

Future spinal reflex is embedded in primary motor cortex output

Mammals can execute intended limb movements despite the fact that spinal reflexes involuntarily modulate muscle activity. To generate appropriate muscle activity, the cortical descending motor output must coordinate with spinal reflexes, yet the underlying neural mechanism remains unclear. We simultaneously recorded activities in motor-related cortical areas, afferent neurons, and forelimb muscles of monkeys performing reaching movements. Motor-related cortical areas, predominantly primary motor cortex (M1), encode subsequent afferent activities attributed to forelimb movement. M1 also encodes a subcomponent of muscle activity evoked by these afferent activities, corresponding to spinal reflexes. Furthermore, selective disruption of the afferent pathway specifically reduced this subcomponent of muscle activity, suggesting that M1 output drives muscle activity not only through direct descending pathways but also through the "transafferent" pathway composed of descending plus subsequent spinal reflex pathways. Thus, M1 provides optimal motor output based on an internal forward model that prospectively computes future spinal reflexes.

Compensatory adaptation of parallel motor pathways promotes skilled forelimb recovery after spinal cord injury

Skilled forelimb patterning is regulated by the corticospinal tract (CST) with support from brainstem regions. When the CST is lesioned, there is a loss of forelimb function; however, if indirect pathways remain intact, rehabilitative training can facilitate recovery. Following spinal cord injury, rehabilitation is thought to enhance the reorganization and plasticity of spared supraspinal-propriospinal circuits, aiding functional recovery. This study focused on the roles of cervical propriospinal interneurons (PNs) and rubrospinal neurons (RNs) in the recovery of reaching and grasping behaviors in rats with bilateral lesions of the CST and dorsal columns at C5. The lesions resulted in a 50% decrease in pellet retrieval, which normalized over four weeks of training. Silencing PNs or RNs after recovery resulted in reduced retrieval success. Notably, silencing both pathways corresponded to greater functional loss, underscoring their parallel contributions to recovery, alongside evidence of CST fiber sprouting in the spinal cord and red nucleus.

Toward biologically realistic models of the motor system

In this issue of Neuron, Chiappa et al.1 describe how neural networks can be trained to perform complex hand motor skills. A key to their approach is curriculum learning, breaking learning into stages, leading to good control.

Supraspinal Plasticity of Axonal Projections From the Motor Cortex After Spinal Cord Injury in Macaques

During recovery following spinal cord injury in the macaque, the sensorimotor cortex on the same side as the injury (ipsilesional, unaffected) becomes activated and plays a role in guiding movements of the affected hand. Effective regulation of these movements by the ipsilesional sensorimotor cortex would depend not only on its ability to send motor commands directly to target muscles but also on coordinated functioning with higher-level motor planning systems such as the cortico-basal ganglia and cortico-cerebellar loops. In this study, using anterograde viral tracers, we analyzed the axonal trajectories of corticofugal fibers from the contralesional (affected) primary motor cortex (M1) at the brainstem level in two macaque monkeys with sub-hemisection spinal cord injury at the mid-cervical level. They showed considerable recovery of grasping movements after injury. We found an increase in axonal projections from the contralesional M1 to the contralateral putamen, ipsilateral lateral reticular nucleus, and contralateral pontine nucleus compared to projections from the ipsilesional (unaffected) M1. We propose that these increased projections from the contralesional M1 to the striatum and precerebellar nuclei on the nondominant side may function to recruit the ipsilesional M1 through the cortico-basal ganglia and cortico-cerebellar loops to control hand movements on the affected side during recovery.

Impairments of inhibitory neurons in amyotrophic lateral sclerosis and frontotemporal dementia

Amyotrophic lateral sclerosis and frontotemporal dementia are two fatal neurodegenerative disorders. They are part of a pathophysiological continuum, displaying clinical, neuropathological, and genetic overlaps. There is compelling evidence that neuronal circuit dysfunction is an early feature of both diseases. Impaired neuronal excitability, imbalanced excitatory and inhibitory influences, and altered functional connectivity have been reported. These phenomena are likely due to combined alterations in the various cellular components involved in the functioning of neuronal networks. This review focuses on one of these cellular components: inhibitory neurons. We assess the evidence for inhibitory neuron impairments in amyotrophic lateral sclerosis and frontotemporal dementia, as well as the mechanisms leading to the loss of inhibition. We also discuss the contributions of these alterations to symptoms, and the potential therapeutic strategies for targeting inhibitory neuron deficits.

Corticospinal and corticoreticulospinal projections have discrete but complementary roles in chronic motor behaviors after stroke

After corticospinal tract (CST) stroke, several motor deficits can emerge in the upper extremity (UE), including diminished muscle strength, motor control, and muscle individuation. Both the ipsilesional CST and contralesional corticoreticulospinal tract (CReST) innervate the paretic UE, but their relationship to motor behaviors after stroke remains uncertain. In this cross-sectional study of 14 chronic stroke and 27 healthy subjects, we examined two questions: whether the ipsilesional CST and contralesional CReST differentially relate to chronic motor behaviors in the paretic arm and hand and whether the severity of motor deficits differs by proximal versus distal location. In the paretic biceps and first dorsal interosseous muscles, we used transcranial magnetic stimulation to measure the projection strengths of the ipsilesional CST and contralesional CReST. We also used quantitative testing to measure strength, motor control, and muscle individuation in each muscle. We found that stroke subjects had muscle strength comparable to healthy subjects but poorer motor control and muscle individuation. In both paretic muscles, stronger ipsilesional CST projections related to better motor control, whereas stronger contralesional CReST projections related to better muscle strength. Stronger CST projections related to better individuation in the biceps alone. The severity of motor control and individuation deficits was comparable in the arm and hand. These findings suggest that the ipsilesional CST and contralesional CReST have specialized but complementary roles in motor behaviors of the paretic arm and hand. They also suggest that deficits in motor control and muscle individuation are not segmentally biased, underscoring the functional extent and efficacy of these pathways.NEW & NOTEWORTHY The corticospinal (CST) and corticoreticulospinal (CReST) tracts are two major descending motor pathways. We examined their relationships to motor behaviors in paretic arm and hand muscles in chronic stroke. Stronger ipsilesional CST projections related to better motor control, whereas stronger contralesional CReST projections related to better muscle strength. Stronger CST projections are also uniquely related to better biceps individuation. These findings support the notion of specialized but complementary contributions of these pathways to human motor function.

Contributions of the Primary Sensorimotor Cortex and Posterior Parietal Cortex to Motor Learning and Transfer

BACKGROUND

Transferring learned manipulations to new manipulation tasks has enabled humans to realize thousands of dexterous object manipulations in daily life. Two-digit grasp and three-digit grasp manipulations require different fingertip forces, and our brain can switch grasp types to ensure good performance according to motor memory. We hypothesized that several brain areas contribute to the execution of the new type of motor according to the motor memory. However, the motor memory mechanisms during this transfer period are still unclear. In the present functional magnetic resonance imaging (fMRI) study, we aimed to investigate the cortical mechanisms involved in motor memory during the transfer phase of learned manipulation tasks.

METHODS

Using a custom-built T-shaped object with an adjustable weight distribution, the participants performed grasp and lift manipulation tasks under different conditions to simulate the learning and transfer phases. The learning phase consisted of four grasp-and-lift repetitions with one motor type, followed by a transfer phase with four repetitions involving different motors (adding or removing a digit).

RESULTS

By comparing brain activity in the learning and transfer phases, we identified three regions (the superior frontal gyrus, supramarginal gyrus, and postcentral gyrus) associated with motor memory during the transfer of learned manipulations.

CONCLUSIONS

Our findings improve the understanding of the role of the posterior parietal cortex in motor memory, highlighting how sensory information from memory and real-time input is integrated to generate novel motor control signals that guide the precise reapplication of control strategies. Furthermore, we believe that these areas contribute to motor learning from motor memory and may serve as key regions of interest for investigating neurodegenerative diseases.

Cortical neuroprosthesis-mediated functional ipsilateral control of locomotion in rats with spinal cord hemisection

Control of voluntary limb movement is predominantly attributed to the contralateral motor cortex. However, increasing evidence suggests the involvement of ipsilateral cortical networks in this process, especially in motor tasks requiring bilateral coordination, such as locomotion. In this study, we combined a unilateral thoracic spinal cord injury (SCI) with a cortical neuroprosthetic approach to investigate the functional role of the ipsilateral motor cortex in rat movement through spared contralesional pathways. Our findings reveal that in all SCI rats, stimulation of the ipsilesional motor cortex promoted a bilateral synergy. This synergy involved the elevation of the contralateral foot along with ipsilateral hindlimb extension. Additionally, in two out of seven animals, stimulation of a sub-region of the hindlimb motor cortex modulated ipsilateral hindlimb flexion. Importantly, ipsilateral cortical stimulation delivered after SCI immediately alleviated multiple locomotor and postural deficits, and this effect persisted after ablation of the homologous motor cortex. These results provide strong evidence of a causal link between cortical activation and precise ipsilateral control of hindlimb movement. This study has significant implications for the development of future neuroprosthetic technology and our understanding of motor control in the context of SCI.

Dynamic lateralization in contralateral-projecting corticospinal neurons during motor learning

Understanding the adaptability of the motor cortex in response to bilateral motor tasks is crucial for advancing our knowledge of neural plasticity and motor learning. Here we aim to investigate the dynamic lateralization of contralateral-projecting corticospinal neurons (cpCSNs) during such tasks. Utilizing in vivo two-photon calcium imaging, we observe cpCSNs in mice performing a "left-right" lever-press task. Our findings reveal heterogeneous populational dynamics in cpCSNs: a marked decrease in activity during ipsilateral motor learning, in contrast to maintained activity during contralateral motor learning. Notably, individual cpCSNs show dynamic shifts in engagement with ipsilateral and contralateral movements, displaying an evolving pattern of activation over successive days. It suggests that cpCSNs exhibit adaptive changes in activation patterns in response to ipsilateral and contralateral movements, highlighting a flexible reorganization during motor learning This reconfiguration underscores the dynamic nature of cortical lateralization in motor learning and offers insights for neuromotor rehabilitation.

Posture-dependent modulation of marmoset cortical motor maps detected via rapid multichannel epidural stimulation

Recent neuroimaging and electrophysiological studies have suggested substantial short-term plasticity in the topographic maps of the primary motor cortex (M1). However, previous methods lack the temporal resolution to detect rapid modulation of these maps, particularly in naturalistic conditions. To address this limitation, we previously developed a rapid stimulation mapping procedure with implanted cortical surface electrodes. In this study, employing our previously established procedure, we examined rapid topographical changes in forelimb M1 motor maps in three awake male marmoset monkeys. The results revealed that although the hotspot (the location in M1 that elicited a forelimb muscle twitch with the lowest stimulus intensity) remained constant across postures, the stimulus intensity required to elicit the forelimb muscle twitch in the perihotspot region and the size of motor representations were posture-dependent. Hindlimb posture was particularly effective in inducing these modulations. The angle of the body axis relative to the gravitational vertical line did not alter the motor maps. These results provide a proof of concept that a rapid stimulation mapping system with chronically implanted cortical electrodes can capture the dynamic regulation of forelimb motor maps in natural conditions. Moreover, they suggest that posture is a crucial variable to be controlled in future studies of motor control and cortical plasticity. Further exploration is warranted into the neural mechanisms regulating forelimb muscle representations in M1 by the hindlimb sensorimotor state.

Baby Observational Selective Control AppRaisal (BabyOSCAR): Scores at 3 months predict functional ability, spastic cerebral palsy distribution, and diagnosis at 2 years

AIM

To assess the predictive capabilities of the Baby Observational Selective Control AppRaisal (BabyOSCAR) tool, administered at 3 months corrected age, in determining spastic cerebral palsy (CP) outcome, functional abilities, and body topography at 2 years of age or later.

METHOD

Independent joint motions were measured at age 10 to 16 weeks from video recordings of spontaneous movement using BabyOSCAR in a sample of 75 infants. All included infants had known 2-year outcomes (45 with spastic CP and 30 without CP) including Gross Motor Functional Classification System (GMFCS) levels and CP body distribution. Receiver operating characteristic curves and cut points indicating greatest sensitivity and specificity were generated for predictive performance.

RESULTS

Total BabyOSCAR score was a strong predictor of future outcome of spastic CP (cut score of 22.5, sensitivity = 98%, specificity = 100%, area under the curve = 0.99), and was able to distinguish children classified in GMFCS levels I and II from those in III to V (cut score of 13.5, sensitivity = 92%, specificity = 89%, area under the curve = 0.94). Having an (absolute) asymmetry score on the BabyOSCAR of more than 5 was a predictor of having unilateral CP at age 2 years (sensitivity = 56%, specificity = 100%, area under the curve = 0.86).

INTERPRETATION

BabyOSCAR scores are predictors of diagnosis, body distribution, and future gross motor function in infants with spastic CP at 2 years of age or later.

WHAT THIS PAPER ADDS

Decreased independent joint movement at 3 months predicts spastic cerebral palsy (CP) at 2 years. Baby Observational Selective Control AppRaisal (BabyOSCAR) scores ≤13 are predictive of Gross Motor Function Classification System (GMFCS) levels III to V. BabyOSCAR scores of 14 to 22 are predictive of GMFCS levels I and II. A BabyOSCAR total asymmetry score >5 predicts unilateral CP. Stereotyped movements are more prominent in those who will be diagnosed with spastic CP at 2 years.

Baby Observational Selective Control AppRaisal (BabyOSCAR): Construct validity and test performance

AIM

To investigate the construct validity of the Baby Observational Selective Control AppRaisal (BabyOSCAR), an assessment of independent joint motion in infants with cerebral palsy (CP).

METHOD

BabyOSCAR was scored for 75 infants (45 with CP and 30 without CP). Rasch analysis was used in combination with classical test theory to assess areas of strength or improvement. Overall fit and precision, unidimensionality, local independence, reliability indices, Wright's child-item map, and differential item functioning were examined as part of Rasch analysis to investigate the item properties, internal construct validity, and reliability of BabyOSCAR. Cronbach's α was used to evaluate items' internal consistency.

RESULTS

Analysis demonstrated good fit to the Rasch model, with only one erratic item. Unidimensionality results suggest two dimensions, split between arm and leg items. Item calibration reliability was between 0.84 and 0.86, with three distinct item difficulty levels. Infant measure reliability was between 0.82 and 0.91, separating infants into three ability levels. Together, the two subscales covered the full range of skills, with redundancy mostly between the same motion on both sides of the body. Cronbach's α was between 0.90 and 0.95.

INTERPRETATION

BabyOSCAR's construct validity was supported. Arm and leg subscales can be translated to a logit scale.

WHAT THIS PAPER ADDS

Baby Observational Selective Control AppRaisal (BabyOSCAR) has excellent construct validity with good overall fit and precision. Individual BabyOSCAR items contribute and work well together, forming an interval-level assessment. BabyOSCAR has two separate subscales, arms and legs, that complement each other. BabyOSCAR's items represent a continuum of skills with three distinct difficulty levels. BabyOSCAR's continuum of skills reliably separates infants into three ability levels.

Baby Observational Selective Control AppRaisal (BabyOSCAR): Convergent and discriminant validity and reliability in infants with and without spastic cerebral palsy

AIM

To describe the development of an observational measure of spontaneous independent joint motion in infants with spastic cerebral palsy (CP), the Baby Observational Selective Control AppRaisal (BabyOSCAR), and to test its convergent validity and reliability.

METHOD

A retrospective sample of 75 infants (45 with spastic CP and 30 without CP) at 3 months of age were scored with the BabyOSCAR and compared with diagnosis of spastic CP, limbs affected, and Gross Motor Function Classification level at 2 years of age or later for convergent validity using t-tests, Kruskal-Wallis tests, and Spearman's rank correlation coefficients. BabyOSCAR interrater and test-retest reliability was also evaluated using intraclass correlation coefficients.

RESULTS

Infants with spastic CP had significantly lower BabyOSCAR scores than children without CP (p < 0.001) and scores were significantly correlated with Gross Motor Function Classification System levels (p < 0.001). Children with unilateral CP had significantly higher asymmetry scores than children with bilateral CP or no CP (p < 0.01). Interrater and test-retest reliabilities were good to excellent.

INTERPRETATION

Reductions in independent joint control measured in infancy are a hallmark of eventual diagnosis of spastic CP, and influence gross motor function later in childhood (with or without a diagnosis of CP).

WHAT THIS PAPER ADDS

Early brain injury causing spastic cerebral palsy results in fewer independent joint movements in infants. Baby Observational Selective Control AppRaisal (BabyOSCAR) score at 3 months depends on limbs affected by early brain injury. BabyOSCAR scores at 3 months correlate with Gross Motor Function Classification System level at ≥2 years. BabyOSCAR has excellent interrater reliability. BabyOSCAR, scored with a 1-minute video recording, has good to excellent test-retest reliability.

Chronic activation of corticospinal tract neurons after pyramidotomy injury enhances neither behavioral recovery nor axonal sprouting

Modulation of neural activity is a promising strategy to influence the growth of axons and improve behavioral recovery after damage to the central nervous system. The benefits of neuromodulation likely depend on optimization across multiple input parameters. Here we used a chemogenetic approach to achieve continuous, long-term elevation of neural activity in murine corticospinal tract (CST) neurons. To specifically target CST neurons, AAV2-retro-DIO-hM3Dq-mCherry or matched mCherry control was injected to the cervical spinal cord of adult Emx1-Cre transgenic mice. Pilot studies verified efficient transgene expression in CST neurons and effective elevation of neural activity as assessed by cFos immunohistochemistry. In subsequent experiments mice were administered either DIO-hM3Dq-mCherry or control DIO-mCherry, were pre-trained on a pellet retrieval task, and then received unilateral pyramidotomy injury to selectively ablate the right CST. Mice then received continual clozapine via drinking water and weekly testing on the pellet retrieval task, followed by cortical injection of a viral tracer to assess cross-midline sprouting by the spared CST. After sacrifice at eight weeks post-injury immunohistochemistry for cFos verified elevated CST activity in hM3Dq-treated animals and immunohistochemistry for PKC-gamma verified unilateral ablation of the CST in all animals. Despite the chronic elevation of CST activity, however, both groups showed similar levels of cross-midline CST sprouting and similar success in the pellet retrieval task. These data indicate that continuous, long-term elevation of activity that is targeted specifically to CST neurons does not affect compensatory sprouting or directed forelimb movements.

A 12-week in-phase bilateral upper limb exercise protocol promoted neuroplastic and clinical changes in people with relapsing remitting multiple sclerosis: A registered report randomized single-case concurrent multiple baseline study

INTRODUCTION

Relapsing-Remitting Multiple Sclerosis manifests various motor symptoms including impairments in corticospinal tract integrity, whose symptoms can be assessed using transcranial magnetic stimulation. Several factors, such as exercise and interlimb coordination, can influence the plastic changes in corticospinal tract. Previous work in healthy and chronic stroke survivors showed that the greatest improvement in corticospinal plasticity occurred during in-phase bilateral exercises of the upper limbs. Altered corticospinal plasticity due to bilateral lesions in the central nervous system is common after Multiple Sclerosis, yet the effect of in-phase bilateral exercise on the bilateral corticospinal plasticity in this cohort remains unclear. Our aim was to investigate the effects of in-phase bilateral exercises on central motor conduction time, motor evoked potential amplitude and latency, motor threshold and clinical measures in people with Relapsing-Remitting Multiple Sclerosis.

METHODS

Five people were randomized and recruited in this single case concurrent multiple baseline design study. The intervention protocol lasted for 12 consecutive weeks (30-60 minutes /session x 3 sessions / week) and included in-phase bilateral upper limb movements, adapted to different sports activities and to functional motor training. To define the functional relation between the intervention and the results, we conducted a visual analysis. If a potential sizeable effect was observed, we subsequently performed a statistical analysis.

RESULTS

Results demonstrated bilateral reduction of the motor threshold alongside with improvement of all clinical measures, but not in any other corticospinal plasticity measures.

CONCLUSION

Our preliminary findings suggest that in-phase bilateral exercise affects motor threshold in people with Relapsing-Remitting Multiple Sclerosis. Therefore, this measure could potentially serve as a proxy for detecting corticospinal plasticity in this cohort. However, future studies with larger sample sizes should validate and potentially establish the effect of in-phase bilateral exercise on the corticospinal plasticity and clinical measures in this cohort.

TRIAL REGISTRATION

Clinical trial registration: ClinicalTrials.gov NCT05367947.

Individual contralesional recruitment in the context of structural reserve in early motor reorganization after stroke

The concept of structural reserve in stroke reorganization assumes that the relevance of the contralesional hemisphere strongly depends on the brain tissue spared by the lesion in the affected hemisphere. Recent studies, however, have indicated that the contralesional hemisphere's impact exhibits region-specific variability with concurrently existing maladaptive and supportive influences. This challenges traditional views, necessitating a nuanced investigation of contralesional motor areas and their interaction with ipsilesional networks. Our study focused on the functional role of contralesional key motor areas and lesion-induced connectome disruption early after stroke. Online TMS data of twenty-five stroke patients was analyzed to disentangle interindividual differences in the functional roles of contralesional primary motor cortex (M1), dorsal premotor cortex (dPMC), and anterior interparietal sulcus (aIPS) for motor function. Connectome-based lesion symptom mapping and corticospinal tract lesion quantification were used to investigate how TMS effects depend on ipsilesional structural network properties. At group and individual levels, TMS interference with contralesional M1 and aIPS but not dPMC led to improved performance early after stroke. At the connectome level, a more disturbing role of contralesional M1 was related to a more severe disruption of the structural integrity of ipsilesional M1 in the affected motor network. In contrast, a detrimental influence of contralesional aIPS was linked to less disruption of the ipsilesional M1 connectivity. Our findings indicate that contralesional areas distinctively interfere with motor performance early after stroke depending on ipsilesional structural integrity, extending the concept of structural reserve to regional specificity in recovery of function.

SpinalTRAQ: A novel volumetric cervical spinal cord atlas identifies the corticospinal tract synaptic projectome in healthy and post-stroke mice

Descending corticospinal tract (CST) connections to the neurons of the cervical spinal cord are vital for performance of forelimb-specific fine motor skills. In rodents, CST axons are almost entirely crossed at the level of the medullary decussation. While specific contralateral axon projections have been well-characterized using anatomic and molecular approaches, the field currently lacks a cohesive imaging modality allowing rapid quantitative assessment of the entire, bilateral cervical cord projectome at the level of individual laminae and cervical levels. This is potentially important as the CST is known to undergo marked structural remodeling in development, injury, and disease. We developed SpinalTRAQ (Spinal cord Tomographic Registration and Automated Quantification), a novel volumetric cervical spinal cord atlas and machine learning-driven microscopy acquisition and analysis pipeline that uses serial two-photon tomography- images to generate unbiased, region-specific quantification of the fluorescent pixels of anterograde AAV-labeled CST pre-synaptic terminals. In adult mice, the CST synaptic projectome densely innervates the contralateral hemicord, particularly in laminae 5 and 7, with sparse, monosynaptic input to motoneurons in lamina 9. Motor pools supplying axial musculature in the upper cervical cord are bilaterally innervated. The remainder of the ipsilateral cord has sparse labeling in a distinct distribution compared to the contralateral side. Following a focal stroke of the motor cortex, there is a complete loss of descending corticospinal axons from the injured side. Consistent with prior reports of axon collateralization, the CST spinal projectome increases at four weeks post-stroke and continues to elevate by six weeks post stroke. At six weeks post-stroke, we observed striking synapse formation in the denervated hemicord from the uninjured CST in a homotopic distribution. Additionally, CST synaptic reinnervation increases in the denervated lamina 9 in nearly all motoneuron pools, exhibiting novel patterns of connectivity. Detailed level- and lamina-specific quantification of the bilateral cervical spinal cord synaptic projectome reveals previously undescribed patterns of CST connectivity in health and injury-related plasticity.

Heightened Reticulospinal Excitability after Severe Corticospinal Damage in Chronic Stroke

OBJECTIVE

After severe corticospinal tract damage poststroke in humans, some recovery of strength and movement proximally is evident. It is possible that alternate motor pathways, such as the reticulospinal tract, may be upregulated to compensate for the loss of corticospinal tract input. We investigated the extent of reticulospinal tract excitability modulation and its inter-dependence on the severity of corticospinal tract damage after stroke in humans.

METHODS

We used a novel startle conditioned transcranial magnetic stimulation paradigm to elicit ipsilateral motor evoked potentials, an index of reticulospinal tract excitability, in 22 chronic stroke participants with mild to severe corticospinal tract damage and 14 neurotypical age-matched controls.

RESULTS

We found that ipsilateral motor evoked potential presence was higher in the paretic arm of people with severe corticospinal tract damage compared to their non-paretic arm, people with mild corticospinal tract damage, and age-matched controls. Interestingly, ipsilateral motor evoked potential presence was correlated with motor impairment across the entire stroke cohort, whereby individuals with worse impairment exhibited more frequent ipsilateral motor evoked potentials (ie, higher reticulospinal tract excitability).

INTERPRETATION

Following severe corticospinal tract damage, upregulated reticulospinal tract activity may compensate for a loss of corticospinal tract input, providing some proximal recovery of isolated and within-synergy movements, but deficits in performing out of synergy movements and finger fractionation remain. Interventions aimed at modulating the reticulospinal tract could be beneficial or detrimental to ameliorating motor impairment depending on the degree of reliance on this pathway for residual motor output. ANN NEUROL 2024.

Ethnokinesiology: towards a neuromechanical understanding of cultural differences in movement

Each individual's movements are sculpted by constant interactions between sensorimotor and sociocultural factors. A theoretical framework grounded in motor control mechanisms articulating how sociocultural and biological signals converge to shape movement is currently missing. Here, we propose a framework for the emerging field of ethnokinesiology aiming to provide a conceptual space and vocabulary to help bring together researchers at this intersection. We offer a first-level schema for generating and testing hypotheses about cultural differences in movement to bridge gaps between the rich observations of cross-cultural movement variations and neurophysiological and biomechanical accounts of movement. We explicitly dissociate two interacting feedback loops that determine culturally relevant movement: one governing sensorimotor tasks regulated by neural signals internal to the body, the other governing ecological tasks generated through actions in the environment producing ecological consequences. A key idea is the emergence of individual-specific and culturally influenced motor concepts in the nervous system, low-dimensional functional mappings between sensorimotor and ecological task spaces. Motor accents arise from perceived differences in motor concept topologies across cultural contexts. We apply the framework to three examples: speech, gait and grasp. Finally, we discuss how ethnokinesiological studies may inform personalized motor skill training and rehabilitation, and challenges moving forward.This article is part of the theme issue 'Minds in movement: embodied cognition in the age of artificial intelligence'.

Learning and Control in Motor Cortex across Cell Types and Scales

The motor cortex is essential for controlling the flexible movements underlying complex behaviors. Behavioral flexibility involves the ability to integrate and refine new movements, thereby expanding an animal's repertoire. This review discusses recent strides in motor learning mechanisms across spatial and temporal scales, describing how neural networks are remodeled at the level of synapses, cell types, and circuits and across time as animals' learn new skills. It highlights how changes at each scale contribute to the evolving structure and function of neural circuits that accompanies the expansion and refinement of motor skills. We review new findings highlighted by advanced imaging techniques that have opened new vistas in optical physiology and neuroanatomy, revealing the complexity and adaptability of motor cortical circuits, crucial for learning and control. At the structural level, we explore the dynamic regulation of dendritic spines mediating corticocortical and thalamocortical inputs to the motor cortex. We delve into the role of perisynaptic astrocyte processes in maintaining synaptic stability during learning. We also examine the functional diversity among pyramidal neuron subtypes, their dendritic computations and unique contributions to single cell and network function. Further, we highlight how cortical activation is characterized by increased consistency and reduced strength as new movements are learned and how external inputs contribute to these changes. Finally, we consider the motor cortex's necessity as movements unfold over long time scales. These insights will continue to drive new research directions, enhancing our understanding of motor cortical circuit transformations that underpin behavioral changes expressed throughout an animal's life.

Dissociative and prioritized modeling of behaviorally relevant neural dynamics using recurrent neural networks

Understanding the dynamical transformation of neural activity to behavior requires new capabilities to nonlinearly model, dissociate and prioritize behaviorally relevant neural dynamics and test hypotheses about the origin of nonlinearity. We present dissociative prioritized analysis of dynamics (DPAD), a nonlinear dynamical modeling approach that enables these capabilities with a multisection neural network architecture and training approach. Analyzing cortical spiking and local field potential activity across four movement tasks, we demonstrate five use-cases. DPAD enabled more accurate neural-behavioral prediction. It identified nonlinear dynamical transformations of local field potentials that were more behavior predictive than traditional power features. Further, DPAD achieved behavior-predictive nonlinear neural dimensionality reduction. It enabled hypothesis testing regarding nonlinearities in neural-behavioral transformation, revealing that, in our datasets, nonlinearities could largely be isolated to the mapping from latent cortical dynamics to behavior. Finally, DPAD extended across continuous, intermittently sampled and categorical behaviors. DPAD provides a powerful tool for nonlinear dynamical modeling and investigation of neural-behavioral data.